The present invention relates generally to gas lasers and, more particularly, to a gas mixture comprising the gain medium within the sealed tube of a carbon dioxide waveguide laser.
The carbon dioxide (CO.sub.2) waveguide laser is typically used as a source of coherent radiation in systems such as an infrared optical radar. In such an application, the properties of the CO.sub.2 laser should include a high output power, so as to permit use at an extended range, and a long sealed-off lifetime, so as to minimize laser maintenance, more particularly, the replenishment of the gain medium gas.
The gas mixture which is initially pumped into a standard CO.sub.2 waveguide laser typically contains carbon dioxide, nitrogen, helium and xenon. The carbon dioxide provides the gain, and the nitrogen provides excitation of the CO.sub.2 by resonant vibrational to vibrational transfer (V-V transfer) of energy by collisions between molecules from its V=1 level. Large amounts of helium are included to help keep the gas cool; if the lower laser level becomes thermally populated the gain and output power of the laser fall. Small amounts of xenon in the gas mixture act as a low ionization potential gas additive.
As soon as the laser is turned on, i.e., the gaseous gain medium of the laser is pumped by electrons produced in a plasma discharge established directly in the gain medium, a significant fraction of the CO.sub.2 dissociates to CO and O.sub.2 through collisions with high energy electrons. If increased amounts of CO.sub.2 are added to compensate for this loss, the temperature of the gain medium increases and the output power does not rise. Indeed, the power falls, since the percentage of helium has been commensurately reduced
Conventional heterogeneous CO-O.sub.2 recombination catalysts, such as a hot platinum filament, are not particularly useful here. These catalysts cannot be placed directly inside the laser bore without adversly affecting the optical quality of the laser resonator and causing a loss of output power. Such catalysts must be placed in an external gas ballast volume and the dissociated gas must then diffuse to this catalyst surface. The effective CO-O.sub.2 recombination rate is then severely reduced by virtue of the long diffusion times, on the order of several minutes.
Homogeneous catalysts are therefore highly desirable for the waveguide laser. Such catalysts work directly in the laser bore where the plasma exists, and the diffusion time for the dissociation products to the catalyst is effectively zero. It is known that very small amounts of hydrogen (H.sub.2) catalyze the recombination of CO and O.sub.2 back to CO.sub.2. However, by itself, H.sub.2 cannot be added in sufficient amounts to retain all of the CO.sub.2, without the output power of the laser dropping off. As an example, adding just one torr of H.sub.2 to a 108 torr gas mix in a radio frequency excited waveguide laser is sufficient to increase the output power from a 4.5-5 watt level occurring with no H.sub.2 to a 8-8.2 watt level. Additional amounts of H.sub.2 begin to cause the output power to decrease, despite further catalyzing the recombination of CO and O.sub.2 so as to increase the amount of CO.sub.2 present.
Another known method to reduce the amount of CO.sub.2 dissociated in the plasma discharge is to replace the N.sub.2 with CO. The vibrational levels of CO and, in particular, the V=1 level, can act to excite the upper laser level of CO.sub.2 by V-V transfer, much as N.sub.2 and CO.sub.2. The presence of excess CO in the gas mix forces the dynamic chemical equilibrium in the plasma back towards the formation of more CO.sub.2 according to the following equation: EQU CO+1/2O.sub.2 +e=CO.sub.2 +e.
This can provide as much as 6.8 watts from the laser of the above example, even though all of the CO.sub.2 does not remain intact. The applicant has found that H.sub.2 in the presence of an excess of CO in the gas mix further returns the amount of CO.sub.2 towards its initial concentration, even better than when H.sub.2 is used in the presence of N.sub.2 without the excess CO. Hydrogen and CO are more effective as a catalyst in combination than either alone. Unfortunately, at any useful concentration, i.e., added H.sub.2 pressures of 0.5 torr or greater in a typical 108 torr gas mix, causes the output power of the laser to drop.
Clearly, there exists a need for a gain medium catalyst to recombine CO and O.sub.2 in a carbon dioxide laser, wherein the catalyst is effective in the recombination process without substantially reducing the laser output power or output efficiency.